Magnetism at the atomic scale is governed by the laws of quantum mechanics, with individual atoms possessing magnetic moments, called “spin,” that are quantized in discrete levels. Even small quantum spin systems, consisting of only a few coupled atoms, can display complex time dynamics. Uncovering this behaviour and learning how to control spins in these model systems may provide insights into larger, more complex systems that are still poorly understood.

It is experimentally challenging to both measure and control atomic spins in solid-state environments. In this thesis, this is achieved using a scanning tunneling microscope (STM), which can be thought of as an atomically sharp needle ending in a single atom. This tip scans over a surface to produce a topographic image that identifies individual atoms. Over the past decade, the capabilities of atomic-resolution scanning probes such as STM have advanced significantly, particularly through the introduction of pump–probe schemes and electron spin resonance (ESR). However, the potential of these techniques has been limited by short coherence times, meaning that spins lose their quantum properties before they can be fully controlled or utilized.

This thesis explores two approaches to improve coherent control of individual spins in on-surface atoms using an STM equipped with ESR capabilities. First, an arbitrary waveform generator (AWG) is integrated into the STM setup to enable faster and more complex voltage signal generation for spin control and readout. Second, the time dynamics of individual nuclear spins are investigated, as these may exhibit longer coherence times than electron spins. A time-resolved readout of the nuclear spin state is achieved via its hyperfine coupling to the electron spin.

The results show that a high-speed AWG can replicate and combine previously separate techniques for coherent single-atom spin control, allowing for more complex experimental designs. In addition, coherent nuclear spin dynamics are observed for the first time with STM, including initial measurements of nuclear spin lifetimes. By compensating for signal distortions, sub-nanosecond pulse widths are achieved, enabling higher time resolution in spin readout. Further experiments reveal nuclear spin lifetimes on the order of seconds—significantly longer than those of other controllable on-surface spins—making single-shot readout possible. Combined experimental and theoretical analysis indicates that the interaction between electron and nuclear spins ultimately limits the nuclear spin lifetime.

Nuclear spins owe their long-lived magnetic states to their excellent isolation from the environment. At the same time, a finite degree of interaction with their surroundings is necessary for reading and writing the spin state. Therefore, detailed knowledge of and control over the atomic environment of a nuclear spin is key to optimizing conditions for quantum information applications. While various platforms enabled single-shot readout of nuclear spins, their direct environments were either unknown or impossible to controllably modify on the atomic scale. Scanning tunneling microscopy (STM), combined with electron spin resonance (ESR), provides atomic-scale information of individual nuclear spins via the hyperfine interaction. Here, we demonstrate single-shot readout of an individual ⁴⁹Ti nuclear spin with an STM. Employing a pulsed measurement scheme, we find its lifetime to be in the order of seconds. Furthermore, we shed light on the pumping and relaxation mechanisms of the nuclear spin by investigating its response to both ESR driving and tunneling current, which is supported by model calculations. These findings give an atomic-scale insight into the nature of nuclear spin relaxation and are relevant for the development of atomically assembled qubit platforms.

The nuclear spin, being much more isolated from the environment than its electronic counterpart, presents opportunities for quantum experiments with prolonged coherence times. Electron spin resonance (ESR) combined with scanning tunnelling microscopy (STM) provides a bottom-up platform to study the fundamental properties of nuclear spins of single atoms on a surface. However, access to the time evolution of nuclear spins remained a challenge. Here, we present an experiment resolving the nanosecond coherent dynamics of a hyperfine-driven flip-flop interaction between the spin of an individual nucleus and that of an orbiting electron. We use the unique local controllability of the magnetic field emanating from the STM probe tip to bring the electron and nuclear spins in tune, as evidenced by a set of avoided level crossings in ESR-STM. Subsequently, we polarize both spins through scattering of tunnelling electrons and measure the resulting free evolution of the coupled spin system using a DC pump-probe scheme. The latter reveals a complex pattern of multiple interfering coherent oscillations, providing unique insight into hyperfine physics on a single atom level.